1. Field of the Invention
The invention relates to the field of static charge neutralization apparatus using corona discharge for gas ion generation. More specifically, the invention is directed to producing electrically self-balanced, bipolar ionized gas flows for charge neutralization. Accordingly, the general objects of the invention are to provide novel systems, methods, apparatus and software of such character.
2. Description of the Related Art
Processes and operations in clean environments are specifically inclined to create and accumulate electrostatic charges on all electrically isolated surfaces. These charges generate undesirable electrical fields, which attract atmospheric aerosols to the surfaces, produce electrical stress in dielectrics, induce currents in semi-conductive and conductive materials, and initiate electrical discharges and EMI in the production environment.
The most efficient way to mediate these electrostatic hazards is to supply ionized gas flows to the charged surfaces. Gas ionization of this type permits effective compensation or neutralization of undesirable charges and, consequently, diminishes contamination, electrical fields, and EMI effects associated with them. One conventional method of producing gas ionization is known as corona discharge. Corona-based ionizers, (see, for example, published patent applications US 20070006478, JP 2007048682) are desirable in that they may be energy and ionization efficient in a small space. However, one known drawback of such corona discharge apparatus is that the high voltage ionizing electrodes/emitters (in the form of sharp points or thin wires) used therein generate undesirable contaminants along with the desired gas ions. Corona discharge may also stimulate the formation of tiny droplets of water vapor, for example, in the ambient air.
Another known drawback of conventional corona discharge apparatus is that the high voltage ionizing electrodes/emitters used therein tend to generate unequal numbers of positive and negative gas ions instead of roughly equal concentrations of positive and negative ions as is desired in most applications. This problem is especially acute in applications requiring the ionization of electropositive gases (such as nitrogen and argon) because high purity electropositive and noble gases have high ionization energy and low electro-negativity. For example, the ionization energy of electronegative O2 is 12.2 eV, compared to 15.6 eV for N2, and 15.8 eV for Argon. As a result, these gases tend to produce large numbers of free electrons rather than negative ions. Restated, although these gases do produce three types of charge carriers (electrons, positive ions, and negative ions) they primarily produce positive polarity ions and electrons. Thus, negative ion emission is relatively rare and the production of positive ions and of negative ions is far from equal/balanced.
Furthermore, ion imbalance may also arise from the fact that ion generation rate and balance are dependent on a number of other factors such as the condition of the ionizing electrode, gas temperature, gas flow composition, etc. For example, it is known in the art that corona discharge gradually erodes both positive and negative ion electrodes and produces contaminant particles from these electrodes. However, positive electrodes usually erode at faster rate than negative electrodes and this exacerbates ion imbalance and ion current instability.
Conventional practice for balancing ion flow is to use a floating (electrically isolated from ground) high voltage DC power supply. The high voltage output of such a power supply is connected to positive and negative electrodes (as shown and described in U.S. Pat. No. 7,042,694). This approach, however, requires using at least two ion electrodes with high voltage isolation between them.
An alternative conventional method of balancing ion flow is to use two (positive and negative) isolated DC/pulse DC voltage power supplies and to adjust the voltage output and/or the voltage duration applied to one or two ion electrodes (as shown and described in published US Applications 2007/0279829 and 2009/0219663). This solution has its own drawbacks. A first drawback is the complexity resulting from the need to control each of the high voltage power supplies. A second drawback is the difficulty of achieving a good mix of positive and negative ions in the gas flow from two separate sources.
The aforementioned problems of emitter erosion and particle generation in conventional ionizers are particularly challenging for corona ionization of high purity nitrogen, argon, and noble gases. Positive polarity corona discharge in these gases generates positive cluster ions having low mobility (low energy) at normal atmospheric conditions. However, negative polarity corona discharge generates high energy electrons as a result of non-elastic collision between electrons and neutral molecules due to field emission from the emitter and photo-ionization in the plasma region around the electrode tip. These free electrons in electropositive and noble gases have a low probability of attachment to neutral gas atoms or molecules. Further, free electrons have more than 100 times higher electrical mobility than gas-borne negative ions. Consequences of these facts include:                high energy electron bombardment of the electrode surface accelerates erosion which, in turn, produces particles that contaminate the ionized gas flow;        high mobility electrons create significant imbalance in the ionized gas flow; and        free electrons are able to produce secondary electron emission, initiate corona current instability and/or cause breakdown.        
One prior art solution to the above-mentioned problems is employed in the MKS/Ion Systems, Nitrogen In-line ionizer model 4210 (u/un). FIG. 1 presents a simplified structure of this apparatus. As shown therein, the ionizing cell (IC) of this device has positive and negative emitters (PE) and (NE) spaced far apart, with gas 3 flowing between them. Each emitter is connected to a floating output of high voltage DC power supply (DC-PS) via current-limiting resistors (CLR1) and (CLR2). In this design, as with others of this general type, positive emitter erosion is a source of contaminant particles and ion imbalance. Also, the efficiency of any system that ionizes a gas stream passing between two electrodes is limited.
Another approach to the same problem is disclosed in U.S. Pat. No. 6,636,411 which suggests introducing a certain percentage of electron-attaching gas (such as oxygen) into the plasma region to convert (attach) free electrons into negative ions and stabilize corona discharge. However, the introduction of oxygen (or some other electronegative gas) precludes use of this approach in clean and ultra-clean environments and/or anywhere non-oxidizing gas streams are required.